1. Field of the Invention
This invention relates to a circuit for converting low frequency AC power from a power grid to high frequency power for starting and operating low pressure gas discharge lamps, e.g., fluorescent lamps.
2. Description of the Prior Art
U.S. Pat. Nos. 4,560,908 ('908) and 4,782,268 ('268) disclose high frequency circuits for starting and operating gas discharge lamps. These are a few of numerous circuits available in the prior art for this function. Such prior art is further described, by way of example, in these patents. The problem addressed is to improve the efficiency of such discharge lamp starting and operating circuits by reducing the harmonic content of the load signal as seen by the power grid. Early prior art circuits use relatively large ballast transformers which exhibit low efficiency. In addition, they introduce distorted signals into the grid power system, namely harmonics of the grid signal frequency. More recently, improved efficiency has been obtained by operating the ballast circuits at a much higher frequency than that of the power grid; for example, typical power grids supply 120 VAC at 60 KHz whereas more recent circuits, such as disclosed in the aforementioned patents, operate generally in the range of about 15 KHz to 50 KHz. These circuits also tend to introduce harmonics, but to a reduced extent than prior circuits.
A purpose of these circuits is to reduce line current harmonic distortion and provide optimum power coupling to the power source. The '908 patent discloses that harmonic distortion above 40% is unacceptable. However, utilities supplying power are now considering implementing rules where harmonic distortion above 20% is unacceptable, especially in the heavily urbanized Northeast United States. Harmonic distortion produced by such a circuit is introduced into the power grid, corrupts the incoming power signal and reduces the efficiency of the power grid accordingly.
In view of the present costliness of supplying electrical power and the inherent environmental problems of constructing new facilities to meet demand, pressure is now on the utilities to improve efficiency. To do so, utilities may mandate improved efficiency of the load circuits driven by the power grid. By way of example, a present standard which the circuit must meet is set by the International Electrotechnical Commission (IEC) in standard 82 which sets forth the permissible harmonic content which may be fed back by a load or user apparatus into a power grid. However, this standard may be changed, at least for part of the United States.
The present invention is directed to a circuit for meeting such a tightened standard by providing reduced harmonic distortion while enabling the circuit to operate with a variety of discharge lamps of differing power requirements. The '268 patent is directed primarily to a circuit which together with the lamp is replaced as a unit while reducing harmonic distortion. The standard to which the '268 patent is directed, however, is present standard 82.
A problem with reducing harmonic distortion in a ballasting circuit is that ideally, minimum harmonic distortion is provided by a circuit current waveform, in a current fed circuit for example, which identically follows the voltage waveform of the power grid signal at the input to the ballast circuit. However, in contrast, the ideal current waveform of a signal for driving a discharge lamp may be a pulsating square wave exhibiting undesirable high harmonic content, and therefore, unacceptable levels of harmonic distortion with regard to the power grid. A high frequency discharge lamp ballast circuit not only must convert a low frequency power grid signal to a much higher frequency, it must compromise two further competing opposing requirements of the power grid and of the discharge high frequency lamp in regard to modulation of the power signal. These opposing requirements necessitate providing a compromise between a modulated and an unmodulated waveform to the lamp, the former being less desirable for the lamp and most desirable for the power grid. The present invention provides a circuit which addresses these conflicting requirements while providing reduced harmonic distortion to meet more stringent requirements.